If my computer says it's compatible ram is 166 or 200 mhz and i put in 400 mhz what will happen

Sometimes it will work at the maximum board allowed amount (200MHz)
Sometimes it won't work and the computer won't turn on
I have never seen it hurt the Motherboard though (possibly the Ram) but not the Motherboard)

K thanks ya it works so hopefully nothing bad will happen\

is the 400Mhz the actual speed or the DDR (effective) speed?

it's the ddr speed is 400 and my motherboard is compatible with 166 or 200 it works but seems slow